Among the variety of services available in the wireless communication systems in general, and mobile communication systems in particular, one which has recently acquired increasing importance is that of efficiently locating the position of a wireless or mobile terminal. By the terms “mobile terminal” it is meant to cover the broadest sense of such terminals such as any of the different known categories of terminals in cellular phone systems such as GSM, GPRS or UMTS, equipment capable of establishing wireless communication while in movement such as mobile computers or personal digital assistants or other similar devices, as well as other moving objects on land, air or sea such as cars, aircraft or ships respectively.
Among known techniques for providing such possibility of locating a mobile terminal, the Global Positioning System (hereinafter GPS) and the Galileo system are known. In the following description, for the sake of simplicity, reference is only made to GPS. Nevertheless it is noted that the invention is not be to be construed as being limited to this system but it can equally be used in other satellite positioning systems such as Galileo.
In very basic terms, GPS uses a constellation of satellites having known coordinates with respect to the Earth. Each of said satellites may transmit, in the form of a broadcast, signals containing information that can be used for determining the position of a mobile terminal. In order to be able to establish a location of the mobile terminal, the information transmitted is in fact received by the mobile terminal from various satellites. As GPS is a well-known technique in mobile terminal positioning applications, further and more detailed description thereupon is not considered necessary.
GPS however suffers from certain drawbacks in mobile terminal positioning procedures. One of such drawbacks is that different layers of atmosphere may introduce fluctuations in the signal transmitted from the satellite. Another problems is occasional lack of visibility of the satellite with respect to the mobile terminal. In the related art, “visibility” is considered to exist where a signal broadcast from a satellite can reach the mobile terminal without being blocked by any obstructing object on the transmission path. Therefore, if the mobile terminal is moved to a region where the visibility of the satellite is blocked, say by a tall building, signals may cease to reach the mobile terminal and thus the latter may not be able to acquire and process the data.
Another drawback associated with GPS applications is the Doppler effect which relates to a change in the frequency of the signal received by the mobile unit with respect to the initial frequency with which it was transmitted. As a consequence, Doppler effect causes certain problems in the accuracy of the received signal, and therefore gives rise to lack of precision in the positioning processing.
In order to overcome the above drawbacks, certain solutions are known. Generally, such solutions are directed to the use of an auxiliary station whose position is fixed and known and is capable of receiving the same signals from the satellite as that received by a mobile terminal in a relatively close vicinity. Therefore, in case of deterioration in the quality of the received signal (as mentioned above) a comparison can be made between the inaccurate results derived from the received signal and the exact position of the auxiliary station which is already known. From such comparison an error factor is obtained which is then transmitted to the mobile terminal. The mobile terminal, which also acquires and processes the same signals, can then take account of such error factor in its own processing of data in order to obtain a more accurate result.
However, this type of solutions suffer from the drawback that they increase the processing requirements in the mobile terminals thus occupying an important part of the resources of the mobile terminal which in turn give rise to an inefficient operation of the terminal.
A known solution to reduce the burden of processing the acquired data on the mobile terminal is the so-called Assisted-GPS (or AGPS). According to the solution provided by AGPS, part of the tasks of acquiring and processing the data broadcast by the satellite is performed in an “assistant” unit which is stationary. According to the solution provided by AGPS, the assistant unit acquires and processes data up to a certain level of completion which otherwise would have been acquired and processed by the mobile terminal. Although the assistant unit and the mobile terminal are not usually in the same area, the assistant unit does have knowledge of the approximate position of the mobile terminal and is capable of combining the information acquired by itself from the satellite with the information related to the approximate position of the mobile terminal and in this manner elaborate part of the information which is useful for being processed by the mobile terminal without the latter having to dedicate its own resources for obtaining the same results.
Although the assistant unit in AGPS systems reduces to a large extent the amount of data processing to be performed by the mobile unit, there still remains substantial processing requirements to be performed by the latter.
In order to better explain the need to reduce said data processing workload reference is again made to the basic data processing in a GPS system. One of the most complex phases in GPS data processing is the acquisition phase. The complexity is due to the nature of the GPS signal which is based on spread spectrum techniques. The acquisition is based on a time (phase) and frequency search of energy related to pseudo-random noise codes. A mobile terminal uses correlation techniques for searching these frequency and phase codes. Once a code is found the procedure is repeated in order to find further codes until all phase and frequency codes are searched and selected. The selection of codes involves an integration process which means a process of integrating each combination of the selected frequency and phase codes in the overall acquired data. As it can be appreciated, the overall searching and selecting procedure for a typical number of frequencies and phase shifts can involve a large number of operations.
The frequency search often suffers from several inaccuracy factors as follows:                instability in the receiver's local oscillator;        Doppler effect caused by the satellites;        Doppler effect caused by the user mobile terminal;        
The first two effects are substantially eliminated if AGPS mode is employed which allows for obtaining assistance data as described above.
The third factor, namely the inaccuracy in user data is highly penalizing because it involves uncertainty in the frequency search while several frequency hypotheses have to be tested for the detection of signal energy. In fact, even when using APGS mode, in order to obtain a good benefit from the assistance data provided by the assistant unit, it is desirable to perform long coherent integration procedures, in the range of 20 ms. This implies the use of relatively very thin frequency slots in the frequency search. For a given frequency uncertainty, the energy research must be made on all the frequency slots which become more and more numerous as the slots become thinner, that is to say:number of the slots=Total freq uncertainty/width of slots
On the other hand it is very difficult to have a priori information on the speed of the user and as a consequence on the user Doppler effect towards each satellite since it require the implementation of complex devices.
In a typical example of acquisition process, the speed of the satellite may be expressed as:{right arrow over (V)}s=(Vx,Vy,Vz);and the induced Doppler may be expressed as:
                    f        0            c        ⁢          (                                    V            →                    s                ·                  u          →                    )        ,where f0 is the central frequency, e.g. 1576 MHz in the case of GPS; {right arrow over (u)} is the unitary vector between the user and the satellite and c is the speed of light. In the case of AGPS, the frequency search is carried out already taking into account the satellite Doppler information. However, there exists the additional uncertainty caused by the user Doppler effect. This implies that the frequency search has to take into account deviation in the information due to said user Doppler effect which is given by the following expression:
                    f        0            c        ⁢                            V          →                u            ·              u        →              ,where {right arrow over (V)}u is the user speed.
In the case of high sensitivity receivers, the acquisition is designed so that the coherent accumulation time (integration) is typically about 20 ms. The span of the frequency slots for searching is inversely proportional to the coherent integration value, namely:
            δ      ⁢                          ⁢      f        =          1              2        ⁢                  T          coh                      ,where Tcoh is the coherent integration duration value.
Therefore for a 20 ms integration time, the frequency slots used are 25 Hz. Supposing that a receiver of a user is moving, for example in a car, at a speed of 100 km/hr, i.e. 27 m/s approximately; the uncertainty due to the user Doppler effect would be approximately 270 Hz for low elevation satellites. This means that, for such a receiver moving horizontally, the acquisition is done over 11 frequency slots (270 user Doppler/25 slot) for satellites presenting a very low elevation angle. This clearly involves a large volume of operations imposed on the mobile terminal.
It is therefore desired to provide a solution according to which positioning data broadcast by a satellite in a GPS or AGPS system may be acquired by or for a mobile terminal in such a manner that the use of resources of the mobile terminal is minimized as much as possible.